This invention relates to electric power converters in general and, in particular, to a single- or multi-phase power converter capable of conversion from one alternating voltage to another in more ways than one; that is, the output voltage may be either equal to, or higher or lower than, the input voltage.
The a.c.-to-d.c.-to-a.c. power converter has been known which is constituted of a half-bridge a.c.-to-d.c. converter and a half-bridge d.c.-to-a.c. inverter. How to improve the efficiency of this type of power converter has also been known, as described and claimed by Japanese Unexamined Patent Publication No. 8-126352 filed by the assignee of the instant application. It suggests use of a high switching frequency, a frequency much higher than that of the input voltage, only for some of the switches included in the a.c.-to-d.c.-to-a.c. converter, and actuation of the other switches at the input voltage frequency for rectification purposes, rather than driving all the switches of the a.c.-to-d.c. converter and d.c.-to-a.c. inverter at the high frequency.
The noted prior art a.c.-to-d.c.-to-a.c. converter is capable of operation in either of three different modes depending upon the way the switches are controlled: (a) Nonconversion Mode in which the output voltage is approximately equal to the input; (b) Stepdown Mode in which the output is lower than the input; and (c) Stepup Mode in which the output is higher than the input. Such a multiway power converter can, in effect, transform one a.c. input voltage into several different a.c. output voltages, or several different a.c. input voltages into one a.c. output voltage.
Although the prior art device succeeds in reduction of switching frequencies, it has some offsetting drawbacks. Its power factor and current waveforms are still unsatisfactory. Additionally, it has very complex circuitry, with too many switches, in order to provide for selective operation in the three different modes.